Hi Helmut,
Let me copy paste my reply on the blog in here for simplicity...
Let me first say that I bought the Denon AH-D600 in that HomeAVDirect sale. I had heard it in a Selfridges store and really quite liked it. So far I am pretty happy with it, but it doesn't stand up to the old Denon D7000 (nor should it). I was tempted by the D7100, but I couldn't quite justify the purchase right now (I've still not been able to hear it myself yet). If you are in the market for this kind of headphone though it seems like a mega bargain so I would strongly consider it, but best test it out for yourself first if you can (Selfridges in Oxford Street have one I think). To help you make your decision I suggest that you read the review for the D7100 on Headfonia:
http://www.headfonia.com/dual-denons-ah-d600-d7100/
From what I read it seems like the D7100 is possibly even more bass heavy than the D7000. Now I love the D7000 for it's huge sub bass and huge wobble, it can be very impressive, but I'm not sure I always want it that strong, or with all my music. As a single headphone to use for everything the Fostex TH600 made a bit more sense to me. It doesn't sacrifice much of the great sensation. It manages to tighten up the bass a lot without loosing much of the impact. You will be able to tell that they are much the same headphone underneath, but the tweaks of the TH600 are mostly for the better. I only wish I could hear the same headphone with a wood back to see if it brought some of that warmth back.
It's worth noting that the D7100 uses a different driver as well as a different design. If you have heard the D7000 and like the sound / comfort it might be best to stick with that if you can't try the alternative before buying them.
From what I can see the D7100 is the same shape / fit as the D600 that I've been using this last week. It's OK on comfort, but nowhere near as good as the D7000 / TH600, which I could wear all day without issue. The D600 has a very narrow headband that causes discomfort on the top of my head after an hour or so because there is too much weight for it's size, although it's worth noting that the top of my head is very sensitive. The cups of the D600 (probably the same as the D7100) are nice and soft, but their shape is not as large as the Fostex models. It barely fits around my ears and feels almost more like the semi-circumaural type that you get on portable headphones where they give you more room on the back than the front. This could be forgivable if they were small, but they're not, they're massive, the Fostex models are a lower profile design. Lastly, but by no means least, the adjustable headband is really odd. It's just about tight enough on me with headband fully collapsed and my head is not small. I honestly think that this design will be too big to fit on some people. Because of the 'Beats' like headband design (one band that goes all the way around both ear cups) they feel really large. I really don't understand why you would do this to such a large headphone, all I can think of is that they sat around a table and thought "hey, Beats sell well, let's copy them".
As for the amps. I love the Schiit range and even the Modi / Magni sound great with the Denons / Fostex, so whatever you can afford will certainly will not disappoint.
PS. The price of the D7100 has gone up to £399 now and the D600 has gone back to the normal price of £199. If you do decide to go back to the TH600 as a prefered option then I recommend you also try the Hifiman HE-500 as well. It's a different sound to either the TH600 of D7000, but this was probably my favourite at this price point. The HE-500s are more mid-centric sound with brighter highs, but the treble (once it calms down a bit / you get used to it) sounds amazingly detailed and this is the point where you realise that planar magnetic bass is something special, not just big, but really detailed and crisp. My only real critisism is that although their deisgn is closer to the TH600 (very comfy / nothing like the D7100's weird fit), but it's heavy metal design can be too much to bare. Anyway, just something to think about if you're looking for one headphone to be great at everything...
Hope this helps
.